The present invention concerns amplifiers for radio frequency (RF) signals and more especially, so-called low noise amplifiers and corresponding receivers.
Conventionally, RF signals are received and processed through a so-called analog front end, part of this processing includes amplifying the signal by low noise amplifiers.
However, the frequency band is always more occupied and signals tend to be closer to each other in frequency. For example, digital television signals extend up to 750 MHz while GSM signals start at 900 MHz. Due to unavoidable non-linearity, the receivers happen to process unwanted signals from nearby frequency ranges.
These unwanted signals are conventionally rejected by use of notch filters which are adapted to reject specified ranges of frequencies. These filters are conventionally placed downstream from the low noise amplifiers.
Accordingly, unwanted signals are still amplified before being rejected. This leads to a sub-optimal amplification as the energy of these unwanted signals are amplified. This leads to lower signal amplification to avoid saturation of the amplifiers and therefore higher noise for the system.
Inverting the order of these components to place the low noise amplifier downstream from the notch filter would lead to a loss of useful signal, as the useful signal is reduced by the filter while the noise is at the same level.
Accordingly, conventional receivers must be designed to balance the loss of useful signals and the sub-optimal amplification.
Some existing devices, such as the one described in the document EP1054510, are designed especially for amplification and have some attenuation as a side effect without any selectivity.